Method of conditioning 18{14 8 stainless steel

ABSTRACT

A method of conditioning type 18-8 stainless steel material for cold work by subjecting the material to a mechanical deforming operation of less than 85% above the Md30 temperature. This processing enables the stainless steel, upon cold work, to transform the austenite to martensite at a much higher rate than in unconditioned stainless steel so that much higher tensile strengths may be achieved with much lower amounts of cold work. In another embodiment of the invention, a method of producing a type 18-8 stainless steel with a tensile strength of over 325,000 psi. at about a 60% cold work level is achieved.

Unite States atent 91 Nunes Mar. 18, 1975 [75] Inventor: John Nunes, Libertyville, Ill.

[73] Assignee: Brunswick Corporation, Skokie, Ill.

[22] Filed: Nov. 29, 1972 [21] Appl. No.: 310,222

[52] US. Cl. 148/12 E, l48/12.4 [51] Int. Cl C22c 39/20, C21d 7/14 [58] Field of Search 148/12, 12.4

[56] References Cited UNITED STATES PATENTS 2,795,519 6/1957 Angel et al. 148/12 3,698,963 10/1972 Nunes et al 148/124 3,752,709 8/1973 Zackay et a]. 148/124 Primary Examiner-W. Stallard Attorney, Agent, or FirmJohn G. Heimovics; Donald S. Olexa; Sheldon L. Epstein [57] ABSTRACT A method of conditioning type 18-8 stainless steel material for cold work by subjecting the material to a mechanical deforming operation of less than 85% above the M temperature. This processing enables the stainless steel, upon cold work, to transform the austenite to martensite' at a much higher rate than in unconditioned stainless steel so that much higher tensile strengths may be achieved with much lower amounts of cold work. In another embodiment of the invention, a method of producing a type 18-8 stainless steel with a tensile strength of over 325,000 psi. at about a 60% cold work level is achieved.

6 Claims, N0 Drawings METHOD OF CONDITIONING 18-8 STAINLESS TEEL.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of high strength stainless steel, and more particularly in the field of making high strength Type 18-8 stainless steel by more economical means.

2. Description of the Prior Art The Type l8-8 variety of stainless steel has for many years been the primary material used when corrosion, was a problem because this material has been proven to be the best all around stainless steel when other qualities, such as formability, strength, ductility, hardness, etc., were also considered as parameters. One of the primary factors that has limited the use of Type l88 stainless steel is that in order to get high strength levels, such as over 350,000 psi, or high hardness levels such as over Rockwell C-55, the material must be subjected to more than 90% cold reduction, Such a high tensile strength l8-8 stainless and a method of achieving such a strength is taught in my US Pat. No. 3,698,963.

Due to tooling, size limitations, geometry, etc., there has been an inabiltiy in a large segment of industry to cold work 18-8 stainless to more than 75% except in fine wire diameters and thin sheet thicknesses. This inability has caused a vacuum indicating the need for a material that does not exist. There have been attempts at creating many new steels that are capable of achieving these desired strength levels, but most do not provide sufficient corrosion resistence. Alternative alloys have been proposed to fill this need which have the high corrosion resistance but lack the sufficient strength. Lastly, some materials have been proposed that fill both the strength requirements and the corrosion needs, but the cost is too prohibitive for general use and application.

In the Angel, et al., US. Pat. No. 2,795,591, a method of processing Type l8-8 stainless utilizing less than 75% cold work is taught; unfortunately, the highest strength level achievable was only 280,000 psi; thus not fulfilling the need for high strength. The 'most desirable solution to achieving high tensile strength and high hardness in an 18-8 stainless steel and yet retaining its corrosion resistance, would be to develop a process whereby the material is subjected to a low level of cold work and yet can achieve these high strengths due to inherent processing properties.

SUMMARY OF THE INVENTION Thus, this invention contemplates a method of providing a conditioned Type 18-8 stainless steel that can be processed by less than 75% cold work and yet achieve tensile strengths in excess of 350,000 psi.

Therefore, it is an object of this invention to provide a process for treating Type l8-8 stainless steel whereby with less than 75% cold work the material can exhibit a tensile strength in excess of 350,000 psi.

It is another object of this invention to provide a process for conditioning such a stainless steel whereby a fabricator can further process such conditioned material to provide end products such as springs, spring wire, wire, fasteners, stampings, etc., that have both high tensile strength and good corrosion resistant properties.

It is another object of this invention to provide a process for treating Type 18-8 stainless steel whereby a lower amount of deformation energy can be employed to get an equivalent strength level resulting in longer tool life and greater forming flexibility.

It is another object of the invention to provide a process for conditioning Type 18-8 stainless steel whereby its metastable state is altered resulting in accelerated martensite formation during subsequent cold working.

The above and other and further objects and teachings of the invention will be more readily understood by reference to the following detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENT Element Percent Carbon .0l.l 5 Chromium l7-l9 Copper 0-.5 Manganese 0-2 Molybdenum 0-.9 Nickel 7- l 0 Phosphorus 0-.04 Silicon 0-l .5 Sulfur 0-.03 Iron Balance Which is substantially the same as the United States Government's AMS Specification. All constituent elements, except for carbon, that are present in less than one percent quantities are considered and characterized as minor elements for the purpose of this disclosure. In addition, the percent cold deformed state, the percent cold worked state,-etc., are all the same as the percent reduction in cross sectional area of the material performed at ambient temperatures. In other words, a 60% cold worked state is the same as a 60% reduction in cross sectional area of the material at ambient temperature, which is not dependent upon any particular type of deformation operation. I

Broadly, the invention contemplates a new process- Warm deformation must take place in a temperaturerange of [50F to 500F above the M temperature, and preferably 200F to 400F, (explained hereinafter). By warm deforming the material at least 50%, and preferably from to less than 15% of the austenite phase is transformed to martensite. The warm deforming highly strains the austenite, conditioning it for a very rapid transformation to martensite when cold deformed. The cold deforming is performed at ambient or room temperature ranging from about 50F to 100F. This conditioning operation gives the material an inherent capability of achieving high tensile strengths with a low amount of cold work.

For a given chemical composition of Type 18-8 stainless steel that has been annealed, at least 85% cold deformation at room temperature is required to obtain a strength level of about 325,000 psi. Yet, for the same chemical composition of a Type 18-8 stainless steel that is both annealed and conditioned as contemplated by this invention, only 60% cold deformation at room temperature is required to achieve the same strength level. For example, in general wire drawing technology, 60% working or reduction in area requires four Brown and Sharpe die passes while 85% working or reduction in area requires eight Brown and Sharpe die passes. The 85% reduction level requires more machine time and causes an increase in labor and tooling costs as compared to the 60% reduction level. Thus, it is obvious that much less energy is required to increase strength of the material and therefore an economic savings can easily be realized.

Once the 18-8 stainless has been thermomechanically treated by first annealing the material and then by warm deformation, as explained above, the material can be shipped to a fabricator who then subjects the material to cold deformation in manufacturing the final product. The strength of this final product can now be approximately 60,000 psi to 100,000 psi greater than unconditioned material without any increase in manufacturing costs.

In the past where a product needed a strength level of at least 375,000 psi but could only be cold worked 75% due to the type of tooling available, the resultant product at 300,000 psi would be unacceptable. But, by the above conditioning process, a product with a minimum of 375,000 psi is achievable with only 75% cold work. Thus, two distinct needs have been filled by this processing; the ability to achieve high tensile strength in areas where the amount of cold work is limited because of tooling, and, secondly, the ability to achieve high tensile strength in a good corrosion resistant material where there are no particular limitations in the amount of cold deformation imparted to the material.

As previously mentioned, the conditioning step requires deformation or working the material at a temperature range of 150F to 500F above the M temperature. T. Angel, in his article entitled Formation of Martensite in Austenitic Stainless Stainless Steel, Journal of the Iron and Steel Institute, Volume 177 (1954) developed an equation that alleges to predict the metastable characteristics (or stability) by chemical composition of Type 18-8 stainless steel materials. By tensile tests, Angel defines the temperature at which 26% deformation work, which equals a strain of 0.3, will produce 50% martensite in a particular chemical composition of material. Since a tensile test does not truly reflect mechanical working deformation such as defined above, the formula for the M temperature was modified by emperical data obtained from actual working conditions with the new formula for the M temperature being:

M temp. 500-462(C+N)2(Si+Mn)12Cr-36- Ni-6Mo wherein C, N, Si, Mn, Cr, Ni and Mo are the weight percentages of carbon, nitrogen, silicon, manganese, chromium, nickel and molybdenum and wherein the temperature is in degrees centigrade. This modified formula has been found very accurate for wire drawing and it is fully contemplated that minor modifications or adjustments may be made thereto for other types of deformation operations such as rolling, extruding, etc. By calculating the specific M temperature for a specific composition of Type l8-8 stainless, the temperature at which the conditioning operation of warm deforming takes place is between 150F and 500F above the calculated M temperature. Many theories and hypotheses can be expounded as to just why this particular range works. However, it is found to work and it is contemplated within the scope of this invention to be the proper working temperature range; the exact temperature being a matter of choice depending upon the type of mechanical working equipment, the desired strength of the final product, etc.

The following are specific examples made in accordance with this invention but should not be construed in any way to limit the scope contemplated by this invention.

EXAMPLE I A type of 302 stainless steel wire having an approximate chemical analysisby weight of:

was solution annealed at a temperature of about 1,950F. Using the above formula, the M temperature was calculated as being 43C or 45F. The wire was then heated to a temperature of 212F, which was 257F above the M temperature, and warm deformed by warm wire drawing to a reduction of of its original size. By standard magnetic methods, the material was found to have 4.9% martensite. The material was then cold worked at room temperature to a 60% level wherein it exhibited a tensile strength of approximately 349,000 psi. The material was further cold worked to a level wherein it exhibited a tensile strength of 440,000 psi. The intermediate stages of cold work levels and tensile strengths are shown on Table l hereinbelow.

EXAMPLE ll The same wire material as Example 1 was solution annealed at l, 950F and then cold worked at room temperature by wire drawing. This material exhibited a tensile strength of 252,000 psi at a 60% cold work level. The material was further wire drawn to a 90% cold work level wherein it exhibited a tensile strength of 354,000 psi. The intermediate cold work levels and tensile strengths at these cold work levels are reflected in Table 1.

EXAMPLE Ill The same wire material as Example 1 was annealed at a temperature of about 1,800F. The wire was then heated to a temperature of 212P, which was 257F above the M temperature of Example I, and deformed by warm wire drawing to a reduction of 75% of its original size. By standard magnetic methods, the ma- EXAMPLE VI The same material as Example V was solution anterial was found to have 7.4% martensite. The material nealed at about 1,950F and then cold worked at room was then cold worked at room temperature to about a temperature by wire drawing. This material exhibited 60% level wherein it exhibited a tensile strength of apa tensile strength of 258,000 psi at a 60% cold work proximately 322,000 psi. The material was further cold level. The material was further wire drawn to a 90% worked to a 90% level wherein it exhibited a tensile cold work level wherein it exhibited a tensile strength strength of about 420,000 psi. The intermediate stages 10 of 370,000 psi. The intermediate cold work levels and of cold work levels and tensile strength are shown on tensile strengths at these cold work levels are reflected Table I. in Table I.

TABLE I Ten Strength in kpsi PERCENT EX. 1 EX. ll IMPROVE- EX. 111 EX. lV IMPROVE- EX. V EX. Vl 1M- PROVE- COLD WORK COND.* UN- MENT COND.* UN- MENT COND.* UN- MENT COND** COND** COND** *Coudiliuncd material Unconditioncd material EXAMPLE IV The same material as Example I was annealed at a temperature of about 1,800F and then cold worked by wire drawing at room temperature. This material exhibited a tensile strength of about 264,000 psi at a 60% cold work level. The material was further wire drawn to a 90% cold work level wherein it exhibited a tensile strength of about 382,000 psi. The intermediate cold work levels and tensile strengths at these cold work levels are reflected in Table I.

EXAMPLE V A type of 302 stainless wire, having an approximate chemical analysis by weight of:

Element Percent Carbon 0.097 Manganese 1.12 Silicon l .24 Chromium 17.22 Nickel 8. l l Molybdenum 0.75 Nitrogen 0.04] Iron Balance 341,000 psi. The material was further cold worked to a 90% level wherein it exhibited a tensile strength of 429,000 psi. The intermediate stages of cold work levels and tensile strengths are shown on Table I.

It can readily be observed from Table I that the material conditioned, as contemplated by this invention, had much higher tensile strength levels than the unconditioned material at each corresponding cold work level. The difference in strength levels at the intermediate cold work levels of 37, 60, 75, and 84% for Examples 1 and [1, indicate about 100,000 psi difference tensile strength, which to one skilled in the art would constitute a grossly significant improvement. Also, the strengths of the 60% cold work level conditioned material are slightly greater than the tensile strengths for the unconditioned materials at the 84% cold work level. Thus, this difference in cold work levels with approximately the same tensile strength indicates that the invention contemplated herein exhibits considerable savings in the amount of cold work energy expended to achieve the same tensile strength and therefore provides a significant economic improvement in prior forms of processing. Ductility measurements made by determining the reduction in area of a fractured tensile specimen are not degraded by the conditioning process.

The low level, under 15%, of martensite developed during the annealing step and the conditioning step, is an indicator that the processing was performed properly; otherwise, the martensite content would be much higher. This can easily be determined by known magnetic measuring techniques. It has been found that the chemical composition of different types of 18-8 stainless may be altered to raise or lower the tensile strength of the material. However, it has been found to be a general rule that the conditioning process contemplated herein provides a material that will yield at least 50,000

psi in strength higher that unconditioned material with the same chemical composition. Thus, the conditioned material can readily be used in the fabrication or wire, strings, fasteners, etc. to provide better material.

Although specific embodiments of the invention have been described, many modifications and changes may be made, especially in minor alterations of chemical composition, without departing from the spirit and the scope of the invention, as defined in the appended claims.

I claim:

1. A process for conditioning Type 18-8 stainless steel for subsequent cold deforming comprising the steps of:

a. annealing the material in a temperature range of from about l,500F to about 2,lF; and,

b. mechanically deforming the material not more than approximately 85% at a temperature of from l50F above the M temperature, thereby c. decreasing the ambient temperature stability of the austenite wherein it forms martensite at a rapid rate upon being cold worked.

2. A process for conditioning type 18-8 stainless steel for subsequent cold deforming comprising the steps of:

a. annealing the material in a temperature range of from about l,500F to about 2,100F; and,

b. mechanically deforming the material not more than approximately 85% at a temperature of from 200F to 400F above the M temperature, thereby c. decreasing the ambient temperature stability of the austenite wherein it forms martensite at a rapid rate upon being cold worked.

3. A process for strengthening Type l8-8 stainless steel comprising the steps of:

a. annealing the material in a temperature range of from about l,500F to about 2,l0OF.

b. mechanically deforming the material not more than approximately at a temperature of from F to 500F above the M temperature as defined in the specification; and

0. cold deforming the material at room temperature at least 60% to produce a tensile strength of at least 325,000 psi therein.

4. The process of claim 3 wherein the mechanically deforming is accomplished at 200F to 400F above the M temperature.

5. The conditioned Type l88 stainless steel formed by the process of claim 1.

6. A high strength l8-8 stainless steel having a tensile strength of at least 325,000 psi made by the process of 

1. A PROCESS FOR CONDITIONING TYPE 18-8 STAINLESS STEEL FOR SUBSEQUENTLY COLD DEFORMING COMPRISING THE STEPS OF: A. ANNEALING THE MATERIAL IN A TEMPERATURE RANGE OF FROM ABOUT 1,500*F, AND, B. MECHANICALLY DEFORMING THE MATERIAL NOT MORE THAN APPROXIMATELY 85% AT A TEMPERATURE OF FROM 150*F ABOVE THE MD30 TEMPERATURE, THEREBY C. DECREASING THE AMBIENT TEMPERATURE STABILITY OF THE AUSTENITE WHEREIN IT FORMS MARTENSITE AT RAPID RATE UPON BEING COLD WORKED.
 2. A process for conditioning type 18-8 stainless steel for subsequent cold deforming comprising the steps of: a. annealing the material in a temperature range of from about 1,500*F to about 2,100*F; and, b. mechanically deforming the material not more than approximately 85% at a temperature of from 200*F to 400*F above the Md30 temperature, thereby c. decreasing the ambient temperature stability of the austenite wherein it forms martensite at a rapid rate upon being cold worked.
 3. A process for strengthening Type 18-8 stainless steel comprising the steps of: a. annealing the material in a temperature range of from about 1,500*F to about 2,100*F. b. mechanically deforming the material not more than approximately 85% at a temperature of from 150*F to 500*F above the Md30 temperature as defined in the specification; and c. cold deforming the material at room temperature at least 60% to produce a tensile strength of at least 325,000 psi therein.
 4. The process of claim 3 wherein the mechanically deforming is accomplished at 200*F to 400*F above the Md30 temperature.
 5. The conditioned Type 18-8 stainless steel formed by the process of claim
 1. 6. A high strength 18-8 stainless steel having a tensile strength of at least 325,000 psi made by the process of claim
 3. 